Nephritis is a general term for inflammation of the kidney, which may be a focal or diffuse proliferative or destructive disease involving the glomerulus, renal tubule or the kidney interstitial (or connective) tissue. Nephritis may progress through a number of stages ending in end-stage kidney disease or end-stage renal failure. The most common form of nephritis is glomerulonephritis.
Glomerulonephritis or glomerular nephritis (“GN”) is a condition characterized by inflammation of the glomeruli or capillary loops of the kidney. The condition occurs in acute, sub-acute and chronic forms and may be idiopathic, or secondary to an infection, disease or exposure to a toxin.
Renal failure is the inability of the kidney to maintain its normal functions. As a result, metabolic waste products and metabolites accumulate in the blood. These waste products and metabolites may adversely affect most bodily systems. Disturbances in the maintenance of fluid and electrolyte balances are characteristic of renal failure.
Acute renal failure may occur suddenly due to trauma, infection, inflammation or exposure to nephrotoxic substances. This condition may result in dehydration, hypotension and circulatory collapse. Acute renal failure is frequently segregated into three categories: (1) pre-renal failure, which is associated with decreased renal blood flow; (2) intra-renal failure, which is associated with ischemia and toxins; and (3) post-renal failure, which results from obstruction of urine flow.
Chronic renal failure involves a progressive loss of kidney function that may eventually progress to end-stage renal disease or failure. At inception, chronic renal failure begins as a diminishing kidney function, without appreciable accumulation of metabolic waste products in the blood. As the glomerular filtration rate slows due to inflammation, waste products begin to accumulate. The disease progresses to uremia due to low kidney function, and high levels of protein end products start to accumulate and impair bodily functions. Common causes of chronic renal failure include: inflammation, infection, urinary tract obstruction and certain systemic diseases and toxicities, including hypercalcemia, lupus erythematosus, diabetes mellitus and hypertension.
End-stage renal disease is marked by irreversible chronic renal failure. Serum creatinine and blood urea nitrogen levels continue to rise and the resulting uremia impairs all bodily systems. The kidney can suffer permanent and almost complete loss of function, on the order of 10% or less of normal kidney function. One cause of end-stage kidney disease is glomerulonephritis. Other causes include those mentioned for chronic renal failure.
Glomeruli are one of the structural components of the nephron of the kidney and are composed of small blood vessels frequently described as a capillary tuft or cluster. The nephron is the basic structural and functional unit of the kidney, which is also comprised of a structure known as a malpighian, or Bowman's, capsule, as well as comprising arterioles and tubules. Bowman's capsule contains the glomeruli loops and the renal tubule. Glomeruli are very small capillaries, hence the blood flow through these vessels is very slow and molecules in the blood can become easily deposited on the walls of these tiny capillaries. Renal tubules are comprised of a basement membrane and epithelial lining and serve to secrete, collect and conduct urine.
The glomerulus functions as a filter within the nephron. Water and small molecules in the blood flow through the glomerulus and are filtered through a structure referred to as the basement membrane, which is formed by the glomerulus and Bowman's capsule. The filtrate comprising water and small molecules passes through the renal tubule to be absorbed and reabsorbed before finally being converted to urine. The basement membrane is comprised of pores of various sizes which serve to filter small molecules and to prevent the passage through the basement membrane of larger molecules. The specific function of the nephron is to remove from the plasma certain end products of metabolism such as uric acid, urea and creatinine as well as excess electrolytes, e.g. sodium, chloride and potassium ions. By reabsorbing water and electrolytes, the nephron plays an important role in maintaining normal fluid balance in the body.
Creatinine is a nitrogenous compound formed as a result of creatine metabolism. Creatine, in turn, is a non-proteinaceous substance that is synthesized in the body from three amino acids, arginine, glycine and methionine. The molecule is found in muscle in small amounts and, when combined with phosphate as phosphocreatine, serves as a storage form of high energy phosphate used in various metabolic processes. Creatinine is absorbed into the blood and ultimately is excreted in the urine. Thus, a simple laboratory test for measuring creatinine in the blood can be used to determine kidney function. The test is frequently referred to as a creatinine clearance test, which measures the amount of creatinine cleared from plasma in a given time interval. Because creatinine is formed from phosphocreatine in relatively constant amounts, a rise in creatinine levels in the blood is indicative of a kidney malfunction, i.e., loss of kidney function.
Glomerulonephritis may arise as a result of a biological insult to the immune system. Foreign substances may adhere to the basement membrane and cause an immune response resulting in the production of antibodies. These antibodies may combine with the foreign substances to cause immune complexes that become deposited on the walls of the tiny glomerular capillaries, resulting in damage to the nephron. Alternatively, in some individuals the immune system can create autoantibodies which are immunoglobulins that may attack kidney cells resulting in a so-called autoimmune response. If proteins in the body are altered, an autoantibody response may ensue because the autoantibodies recognize the altered proteins as non-self. These autoantibody-protein complexes may likewise be deposited on the basement membrane of the glomerulus causing a disruption of the functioning of the nephron.
Glomerulonephritis is a common cause of proteinuria in felines and may be either the idiopathic or secondary form of the condition. In the latter situation, the condition may develop secondary to neoplasia, inflammatory diseases, endocrine malfunctions, infections or familial nephropathies. As in humans, glomerulonephritis in felines is often mediated immunologically, involving immunoglobulins and complement factors in the body of the animal. Injury occurs within the glomeruli of the kidney resulting in morphological changes to the glomeruli. Eventually the injury is irreversible and leads to malfunction of the nephrons.
Glomerulonephritis is characterized in the scientific literature in a number of different forms based on the histopathological changes taking place. Membranous glomerulonephritis involves thickening of the glomerular basement membrane. Proliferative or mesangioproliferative glomerulonephritis is characterized by proliferation of cells in the mesangial matrix. Membranoproliferative glomerulonephritis involves a combination of the foregoing changes. Glomerulosclerosis is characterized by increased matrix formation and scarring. In some cases there are minimal changes to the glomeruli and only slight increases in mesangial cell proliferation.
A number of methods have been developed for studying differential gene expression, e.g., DNA microarrays, expressed tag sequencing (EST), serial analysis of gene expression (SAGE), subtractive hybridization, subtractive cloning and differential display (DD) for mRNA, RNA-arbitrarily primed PCR (RAP-PCR), real-time PCR (RT-PCR), representational difference analysis (RDA), two-dimensional gel electrophoresis, mass spectrometry, and protein microarray based antibody-binding for proteins.
Due to the complexity of the biological pathways implicated in renal disease and the inherent molecular interactions and intercellular signaling processes, it is highly desirable to understand at a genetic level the interactions that are taking place. Detection of dysregulated genes in the early stages of loss of renal function in felines is helpful in understanding the biology of renal disease, especially glomerulonephritis on a genome-wide basis. The fact that gene dysregulation may be detected at an early stage of disease development in animals subjected to repeated ischemic injury is helpful in designing methods for diagnosing of, and devising and monitoring a treatment plan for, an abnormal loss of kidney function, renal failure, reduced glomerular filtration rate or glomerulonephritis, in a feline.
A more detailed understanding of the biological pathways involved through gene expression profiling will aid in the development of diagnostic procedures, reagents and test kits as well as salutary pharmaceutical, nutraceutical and nutritional (dietary) interventions in the disease pathways. These approaches may enable early detection and potentially prevention or treatment of the underlying kidney disorder, particularly glomerulonephritis, as well as in monitoring the prognosis of early stage renal failure and glomerulonephritis, especially in felines. Dysregulated genes involved in the pathology of such disorders may serve as important biomarkers for diagnosis and potentially prevention or treatment of the disorder and to optimize selection of appropriate pharmaceutical, nutraceutical and nutritional (dietary) interventions.
The level of gene expression and/or the determination of the level of functioning of an expressed gene product in a feline may be used to select an appropriate agent for therapeutic or prophylactic use. This data may be employed by the skilled worker in selecting appropriate drugs as agents for the prevention or treatment of renal diseases in felines through gene expression profiling. Gene expression data and analysis may also be used to select nutritional compositions, dietary supplements, and nutraceuticals having a salutary effect on kidney performance by utilizing biomarkers indicative of a healthy state of kidney functioning.
Only very limited work has been done to date in screening the feline genome for gene expression profiles in connection with the diagnosis of diseases in felines. Studies in healthy populations of felines versus populations having a disease such as kidney disease and loss of kidney function as described in this specification have not been extensively conducted. Little data is available with respect to the expression profile of the feline genome, especially with respect to the development of renal diseases in felines over time.
Kidney failure is a leading cause of death in felines. To effectively treat kidney disease, it is important to address the problem early, before the kidney is seriously damaged. By the time the subject is showing signs of kidney failure, the damage may well be irreversible. This presents a challenge, because kidney disease in its early stages may not have any overt symptoms. Accordingly, there is a need for better methods to identify animals in the early stage of kidney disease, so that they can be treated appropriately, for example by giving them appropriate diets in the case of idiopathic conditions, and/or treating conditions such as infection or autoimmune disease which may be contributing to the problem in order to help reverse or at least delay and inhibit the progression of the condition.